Geography Reference
In-Depth Information
sustainable, adult survival must balance reproductive output discounted by
the survival of young birds throughout their i rst year. Because many young
birds disperse, often over considerable distances, distinguishing disappear-
ances during the i rst year that are due to dispersal from those that actually
represent death is daunting. Most ornithologists consider it the “black hole.”
Virtually every estimate of survival before breeding conl ates death with
dispersal and as a result underestimates survival. Dave's estimates were no
exception.
Catching young birds during their i rst summer of life is not so hard.
We caught and banded seven hundred. The problem is i nding them again. We
adequately covered their home turf the next year, spent some time looking in
nearby likely places, and let local citizens know that if they saw a banded bird,
we'd be interested in getting a closer look. With all of this ef ort, we found only
ten of every hundred banded. When Dave analyzed these dismal encounter
histories, he estimated that roughly 20 percent of song sparrows, spotted
towhees, Bewick's wrens, and Swainson's thrushes survived their i rst year. It
was even worse for juncos and Pacii c wrens; they just vanished from most set-
tings. Robins seemed to do better; one in three survived, at least in reserves. If
these estimates were even close to accurate, then few of the populations we
studied were sustainable. We needed another portal into the black hole.
Kara Whittaker held the young song sparrow gingerly. I'm not sure who—
bird or student—was more nervous; both had good reason to be. As Kara
restrained the struggling l edgling, I helped her slip an elastic loop over each
of the bird's legs and position a tiny electronic fanny pack just above the
bird's rump. This miniature radio transmitter weighed just over half a
gram—less than the weight of twenty grains of rice—and would silently tick
